This invention relates to a novel damped strut for attaching an engine to an aircraft. More particularly, the invention relates to a specially adapted engine mount wherein damping elements are located at the strut-airframe interface.
Certain modern unducted fan (UDF) jet engines have two sets of uncowled counter-rotating propeller blades mounted aft of the combustion chambers and turbines. These blades are large with wide chords and thin cross sections. Accordingly, they present a large target area and could be damaged by impact from solid objects. Modern cowled fan jets with very large cross-sections also present large targets for fan blade damage.
Rotating damaged blades at high speeds causes unbalanced forces in the engine which are transmitted to the airframe through the engine attachment structure. The intensity of the unbalance depends on the extent of the damage to the blades.
The flight deck crew or automatic systems ordinarily shut down a damaged engine to reduce the magnitude and duration of such unbalance vibration. However, if attempts to shut down are unsuccessful, the high dynamic unbalance loads might cause damage to the airframe, engine installation or aircraft control elements and impair the ability of the fight crew to perform critical tasks such as reading flight instruments.
Attempts have been made to deal with potentially large engine unbalances by adding elastomeric dampers at or near the location where an engine is mounted to the attachment strut and/or by strengthening selected airframe members. While these measures may have prevented structural damage to an aircraft due to severe unbalance, they did not provide a solution for the large vertical and lateral accelerations transmitted through the airframe and experienced by the crew and passengers.
Since UDF and large fan jet engine failures represent extreme cases of potential damage to an aircraft from engine vibration, this invention was made principally to solve UDF unbalance problems. However, the subject invention has general application to any engine-aircraft combination. It is useful for damping vibration from any engine with a rotating fan, turbine, or propeller such as unducted fan engines, turbofan engines, jet engines, turboprop engines, and piston powered propeller engines. Practice of this invention reduces vibration transmission to the airframe in all circumstances--from ordinary engine operating vibration to severe vibration caused by engine unbalance.
Accordingly, our invention provides a novel means for attaching engines, particularly UDF, helicopter or other large diameter fan engines, to an aircraft which means effectively reduce the adverse effect of vibration and large engine unbalances on the entire structure.